Thanks AHC god @IndroCruise , this is awesome info! I'm guessing it's highly unlikely there's a blockage downstream of the pump assembly? I had just ordered a new pump assembly and will add those other parts you recommend, like you say - if I'm going in there I may as well change it out on a 2006 195k mile rig.
One question - in normal operation, when you bleed the accumulator should you notice a corresponding decrease in the reservoir level or are the two decoupled and what you bleed comes from the pressure on the system side (and hence the car drops).
Finally - when installing the new dry pump with loaded reservoir, is there anything that needs to be done to "prime" the pump or is that not required with a new one?
Good questions -- here are some long detailed answers -- but nothing god-like here, just a few technicalities and opinions ....
1. Is it likely that there can be a blockage downstream of the pump assembly?
This is possible -- but make your own judgement about likelihood.
For example, it is easy to imagine this problem occurring
- if the AHC system has been infected by the batch AHC Fluid containing ‘gel’ (seems mainly to have been related to batches of the 08886-81221 AHC Fluid in one-litre plastic bottles in the US market in 2021/2022), and,
- somehow, albeit unlikely, this gel-infected fluid has found its way through the two strainers inside the AHC Pump sub-assembly, and also,
- after that, this gel-infected fluid has passed through the single strainer inside the Return Valve.
If this could happen, it would cause all kinds of downstream grief at the various orifices, valves and tiny pathways within the components of the AHC system. This downstream passage of 'gel' seems unlikely. However, 'gel' causing blockage of the strainers inside the AHC Pump, particularly the intake strainer, is highly likely -- and such blockages will cause other serious problems as discussed in Post #7.
Poor hydraulic hygiene has a more likely effect --
- where AHC Fluid has not been changed at least as often as set out the Owner’s Manual (every 6 years or 60,000 miles -- and preferably twice as frequently if system longevity is important), and,
- old AHC Fluid is carrying an ever-increasing build-up of oxidation, detritus and sludge throughout the AHC system.
However, an even more common issue is air trapped in the downstream pipework and/or in the body of the ‘shock absorbers’ (which can never be fully flushed while in place on the vehicle -- piston does not and must not travel full length).
Suggest review the hydraulic circuits under various conditions -- see last few pages of the attachment -- also suggest review the descriptions of how the individual components and the whole system is designed to operate.
One of the main points is that the Levelling Valves (in the Control Valve Assembly) are “normally closed” -- this means that
- for most of the time in normal operation, the fluid downstream of these valves cannot move far, unless and until the ECU causes the Levelling Valves to open to allow the AHC Pump (not the Height Control Accumulator) make a small height auto-levelling adjustment, or,
- the driver operates the height switch on the centre console,
- there can be some side-to-side movement of fluid (but not front-to-rear and nor rear-to-front movement) because the Gate Valves (also in the Control Valve Assembly) are “normally open”, except during turns at speed when the ECU causes the Gate Valves to close.
The result of all of those details is that when the AHC system has been disrupted for change-out of major parts, there will be a lot of air to evacuate -- and it may be widely distributed around the AHC system.
It also happens that when aerated AHC Fluid collects in the AHC Tank and the AHC system is in operation
- the AHC Pump then sends this air-polluted AHC Fluid directly to the Height Control Accumulator,
- in turn the Height Control Accumulators sends this fluid all around the system at the next raise, “LO” to “N”, or, “N” to “HI”, and,
- the AHC Pump also sends AHC Fluid all around the system when it is called on to complete a raise or auto-level the vehicle.
Suggest think of this air not as a huge “bubble” along a pipe but as hydraulic fluid (which actually is a light mineral oil with a few added modifiers) and in which air (oxygen and nitrogen) are entrained or dissolved under high pressure and which emerges as fine bubbles when the containing pressure is released -- visible in the receptacle used when bleeding -- and also may be visible in the AHC Tank when the AHC Fluid returns there. This air will release to atmosphere – but very s-l-o-w-l-y!!
The consequence is that bleeding air from an air-polluted AHC system can require multple efforts and repetitive persistence.
The moral of this part of the story is that in AHC diagnosis, before considering more complex problems and solutions ….
- be sure that air is cleared out of the AHC system with persistent multiple bleeding efforts if necessary, and,
- be aware that if old and failing ‘globe’ membrane(s) are slowly but perceptibly leaking nitrogen, then no amount of bleeding will solve the problem until the defective ‘globe’ or ‘globes’ are replaced, or have completely emptied their nitrogen charge into the AHC Fluid,
- at this time it will appear that AHC Fluid has been ‘lost’ because the fluid has now taken over the volume in the ‘globe’ formerly occupied by nitrogen,
- then there will be a very rough springy ride with minimal damping and the “HI/LO Test” of graduation differences at the AHC Tank will give a very poor result).
2. In normal operation, when you bleed the Height Control Accumulator should you notice a corresponding decrease in the reservoir level or are the two decoupled and what you bleed comes from the pressure on the system side (and hence the car drops)?
The Height Control Accumulator has only one function -- to speed up the raise of the vehicle when the driver uses the console switch to move from “LO” to “N”, or, “N” to “HI”.
The Height Control Accumulator is otherwise inactive. It has no role in the “normal running operation” of the vehicle suspension:
When bleeding the Height Control Accumulator correctly with the engine
“OFF”,
@PADDO advised long ago that around 300 millilitres (~0.6 pints) of AHC Fluid should be released. In this case
- the fluid is forced out by nitrogen pressure behind the piston in the Height Control Accumulator,
- the vehicle does not drop because the Levelling Valves are not opened with the engine and AHC system “OFF”,
- If nothing come out, then it means that nothing was in there.
So that tests the assumptions that
(i) the Height Control Accumulator is in healthy condition and its solenoid valve and harness are healthy all the way back to the ECU via the BI1 connector in the LHS Rear quarter panel, and,
(ii) that the Height Control Accumulator as been re-charged correctly in sequence by the AHC Pump, as directed by the ECU, after a prior raise.
If bleeding the Height Control Accumulator incorrectly with the engine
“ON”, then
- the ECU may determine that it should be causing the AHC Pump to attempt to re-charge the Height Control Accumulator and/or,
- attempt to raise the vehicle,
- if so, the ECU then would open the solenoid valve at the front of the Height Control Accumulator,
- AHC Fluid from the AHC Pump would then flow through the solenoid valve and immediately out through the bleeder valve,
- a hydraulic ‘short circuit’ to atmosphere is created!
If the ECU does cause the Levelling Valves to open, then the vehicle could drop.
One or other fault condition (DTC) is likely, and a ‘fail safe function’ would act to protect the system and the vehicle by stopping the action. The Height Control Accumulator would not be re-charged.
If it works, this approach
may be an indirect way of testing the flow from the AHC Pump!!
3. When installing the new dry pump with loaded reservoir, is there anything that needs to be done to "prime" the pump or is that not required with a new one?
When replacing or disrupting the AHC Pump in any way that causes it to lose ‘prime’, the pressure sensor and the ECU will
- detect and act on low pressure and related fault conditions,
- at least one or both of DTC C1751 and DTC C1762 and possibly other DTC's,
- the AHC Pump will stop.
The acknowledged method to re-start the AHC Pump is a brief and careful operation of the “Active Test” -- either via Techstream or using the “manual Active Test” method described in my previous post #7 in this thread.
Some prefer to “inch” the Pump repeatedly against the fault condition, causing it to start, then be stopped by the fault, then repeat the start/stop until the AHC Pump continues to run.
Sometimes, it may be suffiicient just to let the AHC Fluid in the AHC Tank take its time to percolate down and fill the AHC Pump.
Meanwhile, suggest that it is worthwhile to read up on AHC Diagnostics in an LX470 Factory Service Manual. If that is not available, the relevant information is the same as for LC100 and will be found at:
LC100 Workshop Manual - https://lc100e.github.io/
Scroll down the index panel on the LHS of the opening screen:
+ Repair Manual
+ DIAGNOSTICS
+ ACTIVE HEIGHT CONTROL SUSPENSION & SKYHOOK TEMS
Suggest read from the start of this section. The different faults are described, including the precisely specified fault conditions, any ‘fail safe function’ arising from the fault and what that looks like in terms of symptoms, as well as relevant test methods and diagnosis decision trees.
Techstream is vital -- and so is equally attentive observation of the physical symptoms and vehicle behaviour-- both are combined for good diagnosis.
